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Utilizing observations from the Electron Losses and Fields Investigation satellites, we present a statistical study of ∼2,000 events in 2019–2020 characterizing the occurrence in magnetic local time (MLT) and latitude of ≥50 keV electron isotropy boundaries (IBs) and associated electron precipitation. The isotropy boundary of an electron of a given energy is the magnetic latitude poleward of which persistent isotropized pitch angle distributions (Jprec/Jperp ∼ 1) are first observed to occur, interpreted as resulting from magnetic field-line curvature scattering in the equatorial magnetosphere. We find that energetic electron IBs can be well-recognized on the nightside from dusk until dawn, under all geomagnetic activity conditions, with a peak occurrence rate of almost 90% near ∼22 hr in MLT, remaining above 80% from 21 to 01 MLT. The observed IBs span International Geophysical Reference Field (IGRF) magnetic latitudes of 60°–74° with a maximum occurrence between 66° and 71° (L of 6–8), trending toward lower latitudes and premidnight local times with activity. The precipitating energy flux of ≥50 keV electrons averaged over the IB-associated latitudes varies over four orders of magnitude, up to 1 erg/cm2-s, and often includes wide-energy electron spectra exceeding 1 MeV. The IB-associated energies and precipitating fluxes also exhibit peak values near midnight for low activity, shifting toward premidnight for elevated activity. The average total precipitating power deposited over the high-latitude nightside atmosphere (55°–80°; IGRF L ≥ 3) attributed to IBs is 10%–20%, or 10 MW, but at times can approach 100% of the total ≥50 keV electron energy deposition over the entire subauroral and auroral zone region, exceeding 1 GW.
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Categorization of Electron Isotropy Boundary Patterns: ELFIN and POES Observations
Abstract Magnetic field‐line curvature scattering (FLCS) of energetic particles in the equatorial magnetotail results in isotropization of pitch‐angle distributions, loss‐cone filling, and precipitation above a minimum energy at a given latitude. At a fixed energy, the lowest latitude of isotropization is the isotropy boundary (IB) for that energy. Nominally, the IB (latitude) exhibits a characteristic energy dependence due to the monotonic variation of the equatorial magnetic field intensity with radial distance. Deviations from this nominal IB dispersion can occur if the radial variation (spatial or temporal) is non‐mononotic and/or if other precipitation mechanisms prevail. With its sensitive and detailed measurements of electron spectra up to relativistic energies, ELFIN's recent observations reveal a variety of electron IBe patterns near magnetic midnight which are repeatable enough to warrant classification. This study aims to categorize the various IBe patterns observed by ELFIN's high‐fidelity but short lived dataset (a few months), compare them with simultaneous nearby POES observations, which are made with a limited energy coverage and resolution but last for decades, and discuss their possible interpretation. The general agreement between ELFIN and POES IB observations indicate a relatively large‐scale nature of IBe patterns. Surprisingly, there exists a large number (up to 2/3 of all events) of non‐monotonic‐or steep/multiple‐IB patterns. This suggest an abundance of non‐trivial tail current sheet structures or a mixed contribution of two mechanisms in the vicinity of IBe in these cases.
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- Award ID(s):
- 2329897
- PAR ID:
- 10586814
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 12
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024JA033231
